This invention relates generally to semiconductor manufacture, and more particularly to an improved semiconductor component, and to a method for fabricating the component.
Semiconductor components, such as packages, dice and wafers can include terminal contacts, such as contact balls, contact bumps or contact pins. The terminal contacts are in electrical communication with integrated circuits, and other electrical elements, contained on the components. For some components, such as chip scale packages and BGA packages, the terminal contacts can be arranged in a dense grid array, such as a ball grid array (BGA), or a fine ball grid array (FBGA). The terminal contacts provide an input/output capability for a component, and permit the component to be surface mounted to a supporting substrate, such as a printed circuit board (PCB).
FIG. 1A illustrates a prior art semiconductor component 10 having an array of terminal contacts 12 in the form of contact balls. In this example, the component 10 comprises a semiconductor package, such as a chip scale package, a BGA package, or a FBGA package, having a board-on-chip (BOC) configuration. The terminal contacts 12 are typically formed of a solder alloy such as 95% Pb/5% Sn, 60% Pb/40% Sn, 63% Sn/37% Pb, or 62% Pb/36% Sn/2% Ag. Typically, the terminal contacts 12 have the shape of a sphere, a truncated sphere or a hemispherical bump.
In addition to the terminal contacts 12, the component 10 includes an array of bonding pads 14 formed on its backside for attaching the terminal contacts 12 to the component 10. Typically, the bonding pads 14 comprise a solderable metal such as molybdenum, copper or gold. As shown in FIG. 1D, the component 10 also includes conductors 28, wire bonding pads 44, and wire bonds (not shown), that form separate electrical paths between the terminal contacts 12 and the semiconductor die (not shown) contained in the component 10. In the illustrative embodiment, the component 10 also includes plating conductors 42 that facilitate plating of the bonding pads 14 and the wire bonding pads 38. The component 10 also includes a solder mask 18 for protecting and electrically insulating the conductors 28 and the terminal contacts 12. As shown in FIG. 1D, the solder mask 18 includes openings 38 aligned with the bonding pads 14 and with the wire bonding pads 44.
One conventional method for attaching the terminal contacts 12 to the bonding pads 14 uses a solder reflow process. With solder reflow, a layer of eutectic solder is deposited on the bonding pads 14 using a deposition process such as screen printing. A platen can be used to hold the component 10, while the eutectic solder is deposited on the bonding pads 14. Prior to depositing the eutectic solder, a flux (not shown) can be applied to the bonding pads 14 for chemically attacking surface oxides, such that the solder can wet the surfaces to be bonded. The flux also performs a tacking function for the terminal contacts 12 prior to solder reflow. Following application of the flux and eutectic solder, the terminal contacts 12 can be placed on the bonding pads 14 in physical contact with the eutectic solder. A fixture can be used to center and maintain the terminal contacts 12 on the bonding pads 14.
Following placement of the terminal contacts 12 on the bonding pads 14, the component 10 can be placed in a furnace at a temperature sufficient to reflow the eutectic solder and form solder joints 16. The solder joints 16 metallurgically bond the terminal contacts 12 to the bonding pads 14. FIG. 1C clearly shows the solder joints 16 and the terminal contacts 12 bonded to the bonding pads 14. The component 10 can then be removed from the furnace and cooled. As an alternative to a solder reflow performed in a furnace, the bonding process can be performed using a pulse-thermode, a hot-air thermode, or a laser. A solder ball bumper, for example, uses a laser to form the eutectic solder joints 16, and bond the terminal contacts 12 to the bonding pads 14.
Alternately, the terminal contacts 12 can be bonded to the bonding pads 14 by brazing, by welding, or by application of a conductive adhesive.
As shown in FIG. 1A, following the bonding process, the component 10 is typically surface mounted to a supporting substrate 20, such as a printed circuit board (PCB), a FR-4 card, or a module substrate to form an electronic assembly 22. For attaching the component 10 to the substrate 20, additional eutectic solder joints 24 bond the terminal contacts 12 on the component 10 to an array of contact pads 26 on the supporting substrate 20. A solder reflow process, as previously described, can be used to form the solder joints 24, and to bond the terminal contacts 12 to the contact pads 26 on the supporting substrate 20.
One factor that can adversely affect the reliability of the assembly 22 during operation in different environments are fatigue failures of the terminal contacts 12 and the bonding pads 14. Typically, these fatigue failures are induced by thermal expansion mismatches between the component 10 and the supporting substrate 20. For example, if the component 10 comprises a first material, such as ceramic or plastic having a first CTE, and the supporting substrate 20 comprises a second material, such as FR-4 having a second CTE, cyclic loads can be placed on the terminal contacts 12 and on the bonding pads 14 as the assembly 22 is thermally cycled during operation. As shown in FIG. 1C, the forces acting on the terminal contacts 12 and on the bonding pads 14 include tensile forces 31T, moment forces 31M and shear forces 31S.
These forces acting on the terminal contacts 12 and on the bonding pads 14 can also occur during testing of the component following the fabrication process. In particular, semiconductor manufacturers routinely test the components by placement on test boards having sockets for holding the component 10. During these tests the component 10 can be subjected to temperature cycling. As the socket and component 10 typically have different CTEs, cyclic loads as described above, can be placed on the terminal contacts 12 and on the bonding pads 14.
One aspect of the fatigue failures is that some of the terminal contacts 12 and bonding pads 14 are much more likely to fail because they experience the highest loads. FIG. 1B illustrates this phenomena. In FIG. 1B, the relative displacement of the terminal contacts 12 in the X direction is plotted on the left hand Y axis. Nominal shear strain experienced by the terminal contacts 12 is plotted on the right hand Y axis. Also in FIG. 1B, the terminal contacts 12 have been labeled A1-J1 on the X axis. The inner row adjacent to the A1-J1 row would be the A2-J2 row.
Line 30 of FIG. 1B represents nominal shear strain on the terminal contacts 12. Line 32 of FIG. 1B represents relative displacement in the X direction. Line 34 of FIG. 1B represents theoretical displacement were the terminal contacts 12 not soldered to the board 20 (FIG. 1A). As shown in FIG. 1B, the terminal contacts 12, and associated bonding pads 14 on the ends of the component 10 (e.g., A1, J1), move the most in the X direction, and also experience the highest strain. On the other hand, the terminal contacts 12 in the middle of the component (e.g., E1, D1, F1), and their associated bonding pads 14, experience the least movement, and the least amount of strain.
FIGS. 1E and 1F illustrate two possible adverse effects of fatigue failures caused by the forces acting on the terminal contacts 12 and on the bonding pads 14. In FIG. 1E, the bonding pad 14 associated with the A1 terminal contact 12 has separated from the component 10. This situation can cause the conductor 28 which is in electrical communication with the A1 terminal contact 12 to break, preventing signal transmission to and from the A1 terminal contact 12. In FIG. 1F, the bonding pad 14 associated with the A2 terminal contact 12 has separated from the component 10 causing a break 40 in the conductor 28 which is in electrical communication with the A2 terminal contact 12. The break 40 can result from the forces placed on the terminal contact 12, in combination with micro cracks that are introduced during manufacture. The break 40 in the conductor 28 shorts the electrical path, such that signal transmission between the A2 terminal contact 12 and the die (not shown) is not possible.
In view of the foregoing, improved semiconductor components having improved terminal contacts, bonding pads and conductors are needed in the art. The present invention is directed to a semiconductor component in which multiple electrical paths are provided for the terminal contacts, bonding pads and conductors, that are most likely to experience fatigue failures. In addition, the present invention provides a structure that rigidifies and anchors the terminal contacts and bonding pads to the component, such that breaks are less likely to occur.
In accordance with the present invention, an improved semiconductor component, a method for fabricating the component, and an electronic assembly constructed with the component, are provided. The component includes terminal contacts each of which has a primary electrical path for transmitting and receiving signals. In addition, selected terminal contacts on the component have at least one secondary electrical path configured to replace the primary electrical path should it become damaged.
In a first embodiment, the component comprises a board-on-chip semiconductor package having terminal contacts in the form of contact balls in a ball grid array. The component also includes a substrate, a semiconductor die wire bonded to the substrate, and an encapsulant encapsulating the die. The substrate includes an array of bonding pads for the terminal contacts, a pattern of primary conductors in electrical communication with the bonding pads, and wire bonding pads in electrical communication with the conductors. The substrate also includes a solder mask covering the conductors, and having openings aligned with the bonding pads and with the wire bonding pads.
Selected terminal contacts, such as the terminal contacts on the outer edges or corners of the array, are in electrical communication with secondary conductors configured to provide alternate electrical paths to the selected terminal contacts equivalent to the primary electrical paths. With this arrangement, an alternate electrical path becomes operable should damage occur to a primary conductor due to thermally induced loads or other factors. In addition, the secondary conductors are configured to rigidify and anchor the selected terminal contacts and their bonding pads, such that separation from the substrate during loading is less likely to occur.
The method for fabricating the component includes the step of providing the substrate having the bonding pads, the primary conductors in electrical communication with the bonding pads, and the wire bonding pads in electrical communication with the conductors. In addition, the method includes the step of providing secondary electrical conductors in electrical communication with the selected bonding pads. The method also includes the steps of attaching the die to the substrate, wire bonding the die to wire bonding pads, and then bonding the terminal contacts to the bonding pads.
The electronic assembly includes one or more components bonded to a supporting substrate, such as a module substrate or a circuit board. The supporting substrate also includes contact pads configured for bonding to the terminal contacts on the components. During use of the electronic assembly, alternate electrical paths are provided to the selected terminal contacts on the components should damage occur to the primary electrical paths. The alternate electrical paths allow signals to be transmitted to and from the affected terminal contact.
In an alternate embodiment of the invention, the component comprises a bumped semiconductor die or a bumped semiconductor wafer. In another alternate embodiment the component comprises a semiconductor package having terminal contacts in the form of contact columns in a column grid array. In each of the alternate embodiments both primary conductors and secondary conductors are provided to selected terminal contacts.